The present invention relates to an electrostatic ink jet record head and an ink jet recorder using the same. More particularly, the present invention is concerned with an electrostatic ink jet record head of the type using ink consisting of a carrier liquid and toner particles dispersed therein, and causing only the toner particles to fly electrostatically so as to print an image on a recording medium, and an ink jet recorder using the same.
Nonimpact recording methods are attracting increasing attention because they produce only a negligible degree of noise ascribable to operation. Particularly, an ink jet recorder which is a specific form of a nonimpact recorder has a simple construction and high-speed recording capability and is operable with plain papers. The ink jet recorder with such advantages has been proposed in various forms in the past. One of conventional ink jet recorders uses ink consisting of a carrier liquid and toner particles dispersed therein. In this type of recorder, a voltage is applied to between a needle-like ejection electrode and a counter electrode facing it with the intermediary of a paper. The resulting electric field causes the toner particles of the ink to fly with an electrostatic force and form a dot on a paper or similar recording medium.
Specifically, the above ink jet recorder has an ejection port in the form of a gap small enough to form an ink meniscus. An ejection electrode is positioned in the ejection port and slightly protrudes to the outside from the end of the port. An electrophoresis electrode surrounds an ink chamber. A counter electrode is connected to ground and positioned on the imaginary extension of the ejection electrode. A paper intervenes between the ejection electrode and the counter electrode. Therefore, in the event of recording, an electric field is formed between the two electrodes. The electric field concentrates on the sharp tip of the ejection electrode and extends toward the paper with high intensity. To effect high-speed recording with the above recorder, it is necessary to replenish the toner particles at a high speed, i.e., to increase the speed of electrophoresis of the toner particles. To increase the speed of electrophoresis speed, it is necessary to apply a great potential difference between the ejection electrode and the electrophoresis electrode. However, because the electrophoresis electrode remains in electrical contact with the ink, even the ink around the ejection electrode has the same potential as the electrophoresis electrode in an equilibrium condition. Therefore, if the high voltage is applied to the electrophoresis electrode excessively, then the potential of the ink around the ejection electrode becomes high enough for the toner particles to fly. As a result, the toner particles fly by themselves even when no drive pulses are applied to the ejection electrode.
To better understand the present invention, a brief reference will be made to a conventional electrostatic ink jet recorder, shown in FIGS. 1-3. Briefly, the recorder uses ink consisting of a carrier liquid and toner particles dispersed therein, and includes a needle-like ejection electrode and counter electrode facing it with the intermediary of a paper or similar recording medium. A voltage is applied to between the ejection electrode and the counter electrode so as to generate an electric field. The toner particles of the ink are caused to fly by the electrostatic force of the electric field forming an image on the paper.
As shown, a head 50 has an ink chamber 52 delimited by a lower plate 63, a side wall 64, and an upper plate 65. A pump, not shown, constantly circulates ink 51 in the chamber 52 via circulation ports 59 and 60. The ink 51 has the above-mentioned composition. An ejection port 54 is formed in a part of the side wall 64 and has a gap small enough to form an ink meniscus Me. An ejection electrode 55 is positioned in the ejection port 54 and slightly protrudes to the outside from the end of the port 54. The surface of the electrode 55 is coated with an insulator to be insulated from the ink 51 thereby. An electrophoresis electrode 58 delimits the other three sides of the ink chamber 52 where the ejection port 54 is absent. The electrode 58 is partly positioned in the chamber 52 and held in contact with the ink 51.
The ejection electrode 55 is connected to a driver, not shown. In the event of recording, a high-voltage pulse of the same polarity as the toner particles is applied to the electrode 55. A high voltage of the same polarity as the toner particles is continuously applied to the electrophoresis electrode 58 from a voltage controller 62. A counter electrode 61 is connected to ground and positioned on the imaginary extension of the electrode 55. A paper P intervenes between the electrodes 55 and 61. Therefore, in the event of recording, an electric field is formed between the electrodes 55 and 61. Because the electrode 55 has a sharp tip, the electric field concentrates on the tip of the electrode 55 and extends toward the paper P with high intensity. The toner particles dispersed in the ink 51 have been charged by zeta potential beforehand, so that they are pulled toward the paper P by a Coulomb's force derived from the above electric field. When the Coulomb's force overcomes the surface potential of the ink 51, the toner particles are caused to fly toward the counter electrode 61 in the form of a drop 53. The drop 53 deposits on the paper P and forms a dot thereon. In this type of recorder, the high-voltage pulse to be applied to the ejection electrode 55 is controllable in accordance with an image to be printed on the paper P.
Just after the flight of the toner particles, i.e. drop 53, the toner content of the ink 51 becomes low in the vicinity of the ejection electrode 55 because only the toner particles are mainly consumed. However, the high-potential continuously applied to the electrophoresis electrode 58 causes the toner particles in the ink 52 to electrophoretically migrate toward the electrode 55 away from the electrode 58. Consequently, only the toner particles are replenished to a portion around the electrode 55. Particularly, because the electrode 55 is electrically insulated from the ink 51, the migration of the charged toner particles toward the electrode 55 ends as soon as the potential distribution in the chamber 52 reaches equilibrium. Therefore, the recordable frequency of the head 50 is determined by the period of time necessary for the toner particles to migrate.
To effect high-speed recording with the above recorder, it is necessary to replenish the toner particles at a high speed, I.e., to increase the speed of electrophoresis of the toner particles. Assume that the amount of charge deposited on the toner particles is q, that the electric field is E, that the ink has a viscosity of .eta., and that the toner particles have a diameter of r. Then, an electrophoresis speed .nu. is expressed as: EQU .nu.=qE/6.pi..eta.r
Because the above factors q, .eta. and r are fixed values particular to the ink, E must be increased in order to increase the electrophoresis speed .nu.. That is, it is necessary to apply a great potential difference between the discharge electrode 55 and the electrophoresis electrode 58. However, because the electrode 58 remains in contact with the ink 51, even the ink 51 around the electrode 55 has the same potential as the electrode 58 in the above equilibrium condition. Therefore, if the high voltage is applied to the electrode 58 excessively, then the potential of the ink 51 around the electrode 55 becomes high enough for the toner particles to fly. As a result, the toner particles fly by themselves even when no drive pulses are applied to the electrode 55.